The invention relates to a method of fabricating a semiconductor device including an improved polycrystalline silicon film, and more particularly to a method of forming an improved polycrystalline silicon film having a reduced resistivity and a good step coverage characteristic.
The method of forming semiconductor devices including an impurity doped polycrystalline silicon thin film are well known in the art. Generally, an impurity doped polycrystalline silicon film may be formed by the prior art fabrication method which will be described. A silicon substrate is prepared to form a semiconductor device having a polycrystalline silicon film. A silicon oxide film is formed on the silicon substrate. Subsequently, an amorphous silicon film is deposited on the silicon oxide film by using a deposition method such as a chemical vapor deposition (CVD).
In the deposition of the amorphous silicon film, a growth gas such as a silane (SiH.sub.4) gas or a silane based gas is used, but also an impurity gas such as phosphine (PH.sub.3) is used to introduce an impurity such as phosphorus (P) into amorphus silicon so that the impurity doped amorphous silicon film may be deposited FIG. 1 illustrates a characteristic of introduction of an impurity gas, namely a flow rate of the impurity gas during the deposition of the amphorus silicon film in which the phosphine gas is diluted with a nitrogen (N.sub.2) gas up to 0.1%. From FIG. 1 of the prior art, it is understood that the flow rate of the impurity gas remains at a predetermined constant value, for instance, 6 sccm (standard cubic centimeter per minute) during the deposition of the amorphous silicon film.
Subsequently, the impurity doped amorphous silicon film is subjected to a heat treatment at a temperature in the range from 600.degree. to 1000 .degree. C. thereby resulting in a crystallization of amorphus silicon to form an impurity doped polycrystalline silicon film. Such arts are disclosed in T. Kobayashi et al. Extended Abstracts of the 20th (1988 International) Conference on Solid State Devices and Materials, Tokyo, 1988, pp. 57.
Such polycrystalline silicon films formed by the above prior art method include crystal grains having a size of 1 to 2 micrometers. While the crystal grain size may be varied by conditions such as a temperature of a heat treatment, it is limited to the size in the above range from 1 to 2 micrometers. Then, it is difficult to produce larger crystal grains. The resistivity of the impurity doped polycrystalline silicon film tends to depend upon the crystal grain size. The realization of reduction of the resistivity requires a larger crystal grain size. It is, therefore, undesirable to apply such polycrystalline silicon film to devices such as thin film transistors which require a crystal grain size of several micrometers for possession of the reduced resistivity.
With regard to a step coverage characteristic, such impurity doped polycrystalline silicon films possess an inferior step coverage characteristic as compared with impurity undoped polycrystalline silicon films. As a result, it is difficult to use those films as a capacitive electrode in semiconductor devices. If those films are used, capacitance characteristics of the capacitive electrodes are substantially deteriorated.